1. Field of the Invention
This invention relates to a process for blown film extrusion. More particularly, the invention relates to an improvement whereby high modulus thermoplastic films can be extruded without wrinkles by a process including the use of a cylinder mounted above the air ring.
2. Description of Related Art
Extrusion of film through a circular die has been practiced for over three decades, and need not be described in detail here. The most commonly blown resins include low density polyethylene (LDPE), blends of linear low density polyethylene (LLDPE) with LDPE, high density polyethylene (HDPE), poly(vinyl chloride) (PVC), and nylon. Because these resins differ in rheology, heat transfer and other physical properties, they behave differently when blown into film. Processing equipment designed or optimized for one resin, such as for LDPE, will often be unsuitable for processing other resins.
In blown film extrusion, the resin forms a tube of molten polymer as it travels upward from a circular die. Air inside this tube forces it to expand radially outward as it cools, forming a film "bubble." The bubble is then flattened by nip rollers or other devices which serve to trap a fixed amount of air within the bubble.
The position at which the molten polymer solidifies is called the frost line. Above the frost line, there is usually no further expansion and the bubble has achieved its final diameter.
Cooling devices called air rings are typically included above the die to cool the molten film more rapidly. There are many variations in air ring designs. Many have two or more air openings such as the so-called "dual lip" air rings, although rings with a single air orifice are also suitable. An alternative is tandem air rings, such as shown in U.S. Pat. No. 4,330,501 to Jones et al., wherein forced air is provided at two levels.
With large volumes of air being introduced at high velocities, processing problems can arise. The film bubble can be unstable, drifting or "snaking" from side to side, producing gauge variations in the film. This is often a concern when blowing "long stalk" films; i.e., those in which the bubble expands after the film travels a relatively great distance above the die.
Resins which are blown "in the pocket," i.e., those in which the bubble expands radially only a short distance above the die, can suffer from air impinging the film at a high velocity. In severe cases the air can deform the bubble at a critical phase of cooling and wrinkles will be produced in the film. This is a particular problem when blowing films from high modulus resins, either in a monolayer film or in a multilayer film with one or more other resins.
Similar difficulties encountered with other resins have spurred experimentation with new devices for providing or channeling the cooling air. U.S. Pat. No. 4,447,387 to Blakeslee, III, et al. suggests the use of a passive flow air ring or "air collar" located above the primary air ring. Forced air is provided to the lower air ring, while a second source of cooling air is induced to flow through the induced flow air collar. Also mounted on the upper surface of the induced flow air collar is a gas flow rectifier or diffuser as shown by numbers 44 and 46 of the figure in that patent.
Another variation is described in U.S. Pat. No. 4,399,094 to Fujutani et al. wherein a tube of linear low density polyethylene passes through a first enclosed space, then passes an air ring where it is contacted with cold air for rapid cooling, then through a second enclosed space where it is allowed to expand and solidify. According to Example 1, the position of the frost line is immediately above the air ring.
In spite of these proposed equipment modifications, there remained a need for a system capable of processing high modulus resins without producing bubble deformation and wrinkles in the film. It is therefore an object of this invention to provide a process capable of overcoming these difficulties without extensive equipment alteration.